Pursuant to the goal of making polyethylene terephthalate(PET) and other polyesters, a great deal of patent literature is dedicated to the describing processes for preparing a carboxylic acid/diol mixture suitable as starting material. In general, these inventions describe specific mixing schemes with a purified terephthalic acid solid and liquid ethylene glycol. Additionally, there is substantial body of literature devoted to producing a purified terephthalic acid in the powder form that is suitable for use in producing PET and other polyesters.
The objective of this invention is to describe a process by which the carboxylic acid/diol mixture suitable as a starting material for polyester production is obtained from a carboxylic acid/solvent slurry without isolation of a substantially dry carboxylic acid solid. More specifically, the objective of this invention is to describe a process by which a terephthalic acid/diol mixture suitable as a starting material for polyester production is obtained from a terephthalic acid/solvent slurry without isolation of a substantially dry terephthalic acid solid.
Usually, purified terephthalic acid solid is produced in a multi-step process wherein a crude terephthalic acid is produced. The crude terephthalic acid does not have sufficient quality for direct use as starting material in commercial PET. Instead, the crude terephthalic acid is usually refined to purified terephthalic acid solid.
Liquid phase oxidation of p-xylene produces crude terephthalic acid. The crude terephthalic acid is dissolved in water and hydrogenated for the purpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is a more water-soluble derivative, and for the purpose of converting characteristically yellow compounds to colorless derivatives. Any 4-carboxybenzaldehyde and p-toluic acid in the final purified terephthalic acid product is particularly detrimental to polymerization processes as they act as a chain terminator during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET. Typical purified terephthalic acid contains on a weight basis less than 25 parts per million (ppm) 4-carboxybenzaldehyde and less than 150 ppm p-toluic acid.
A number of other processes have been developed where a terephthalic acid suitable as starting material for commercial PET production without the use of hydrogenation. Typically, these processes usually involve catalyzed oxidation of p-xylene in an acetic acid solvent followed by filtration and drying of the terephthalic acid from the acetic acid solvent.
To produce a terephthalic acid/diol mixture acceptable for PET production from a terephthalic acid/solvent slurry poses a substantially different problem than from a terephthalic acid and water mixture.
Typically, TPA produced via catalyzed oxidation of p-xylene in an acetic acid solvent produces a terephthalic acid/solvent slurry that contains residual catalyst (e.g cobalt, manganese, and bromine). In a common method of producing a substantially dry TPA solid from a terephthalic acid/solvent slurry, the terephthalic acid/solvent slurry is filtered to separate a substantial amount of the acetic acid liquid from the TPA solids. Residual catalyst is usually separated from the terephthalic acid/solvent slurry by washing (rinsing) the wet cake with catalyst-free acetic acid, water or other solvent. The TPA solid is isolated by drying.
In the present invention, a novel process has been discovered resulting in fewer steps than the currently employed processes. The primary utility of the invention is reduction of capital and operating costs associated with the isolation of a terephthalic acid powder. In the conventional approach toward producing terephthalic acid via catalyzed oxidation of p-xylene in an acetic acid solvent, a terephthalic acid/solvent slurry is filtered, washed, then dried to produce a terephthalic acid powder suitable as starting material for PET production.
In one embodiment of the present invention, the terephthalic acid/solvent slurry is filtered to produce a terephthalic acid cake with solvent and a TPA solvent mother liquor stream. The terephthalic acid cake with solvent is then washed (rinsed) with water to recover residual metal catalyst material and to produce a water-wet terephthalic acid cake and an TPA solvent/water by-product liquor. The water-wet terephthalic acid cake is then combined with a diol to produce a terephthalic acid/diol mixture suitable as starting material in a commercial PET process. By bypassing conventional processes for isolating a terephthalic acid solid, the equipment necessary produce a terephthalic acid powder is eliminated.
Another surprising and seemingly contradictory aspect of the invention is the benefit of addition of water to the acetic acid and ethylene glycol solvents. In general, in conventional processes for producing terephthalic acid, it is necessary to remove water produced in the oxidation process. Typically, use of acetic acid as an oxidation solvent necessitates an additional process step where acetic acid and water are separated. It is seemingly contradictory to produce an acetic acid and water mixture when it can be avoided by drying the terephthalic acid from the acetic acid solvent.
Additionally, in processes for producing PET via esterification of TPA with ethylene glycol, water is generated as a reaction by-product. In general, it is necessary to remove the water produced in the esterification process via an additional process step where ethylene glycol and water are separated. It is seemingly contradictory to produce an ethylene glycol and water mixture when it can be avoided by not introducing water with the TPA.
However, the one benefit of this invention is based on the premise that ethylene glycol/water and acetic acid/water separation systems normally exist for conventional TPA and PET production processes. In this invention, the savings associated with eliminating the TPA drying is of greater benefit than the incremental increase in ethylene glycol and water separation capacity plus the incremental increase in acetic acid and water separation capacity.